1. Field
The invention relates to the field of guidance control systems, providing a method and apparatus for dynamically guiding a controlled item such as a guided missile or guided projectile, toward intersection with the target at a location such as a point along a potentially changing trajectory of the target.
2. Related Art
Various guidance systems are possible for directing a guided missile, projectile or other moving item toward a destination by use of corrective lateral acceleration. In a case where the destination is a stationary point, there may be uncertainty in fixing the location of the target, which advantageously is decreased as the missile or projectile approaches the target. The trajectory of the guided item may be influenced by external factors, requiring guidance corrections. Where the destination is a predicted point of intersection with the trajectory of a moving object, the trajectory of the object may change as well, requiring guidance corrections.
A guidance system typically is coupled to sensing inputs from which up-to-date data is obtained in one way or another regarding the location and motion of the target. The guidance system is configured to control variable output devices for establishing an appropriate trajectory of the guided item to intersect with the target. A controller determines the amplitude of acceleration to be applied by the output devices to achieve the desired result. For example, a proportional control may apply lateral acceleration as a function of the error between the predicted positions of the guided missile and the target at a distance ahead and at a future point in time. In a weapons system, for example, the desired result may be the intersection of the trajectories of the guided item, such as a guided missile, with an independently guided target such as an aircraft. A self-propelled guided missile is used as an example in this description. It should be appreciated that the missile could be an artillery round or an object falling from an aircraft.
A guidance controller determines the acceleration to be applied by the output devices using a guidance scheme or guidance law, for example embodied in the programming of a processor or other circuitry associated with the controller. An advantageous guidance scheme is to apply acceleration in an amount that is related, according to the guidance law, to the error between the current trajectory and the trajectory that will cause the desired result, such as intersection of the trajectory of the missile with the trajectory of the target at the earliest possible time.
As a non-limiting example, a two dimensional guidance rule might predict a point of intersection from the progress of a line of sight to the target versus continuation of the missile along its current heading. In another example, a range to the target may be known such that plotting the trajectory of the missile and that of the target can be done in three dimensional space. Whether calculated in two dimensions or three, guidance corrections to the trajectory of the missile may be advantageous, such as lateral acceleration impulses in a direction that is perpendicular to the velocity vector of the missile or perpendicular to a line of sight toward one or more of the target and the predicted point of intersection. The missile is steered.
An error between the current missile trajectory versus a desired missile trajectory to intersect with the target, is identified when the bearing to the target differs from the expected bearing to the target, leading to a conclusion that the expected bearing to the point of intersection is not accurate. The guidance law re-computes the point of intersection and applies lateral acceleration to alter the trajectory of the missile, e.g., in a direction perpendicular to a line of sight from the missile to the expected new point of intersection. Lateral acceleration for a time adjusts the missile velocity vector toward crossing the target trajectory at the new point of intersection.
Different sorts of targets might be stationary, or on a fixed velocity vector, or actively accelerating, decelerating and/or steered laterally in one direction or another (in this context, “lateral” encompasses up, down, left or right). The trajectory of the target and/or the guided item may be affected by outside influences such as windage, gravitational acceleration on a parabolic path or similar factors. In any case, the controller estimates the target trajectory and guides the missile trajectory to intersect the target.
Mathematical guidance laws have been developed to operate on inputs that contain knowledge of the states of the missile and the target. The success of a guidance law turns on the accuracy of knowledge of the states of the missile and the target. There is some inherent error or tolerance in the accuracy of such knowledge, for example, the relative bearing of the target, or its range or velocity vector, or the like. There also is some inherent error or tolerance in the guidance control outputs.
In a typical guidance system, inherent input and output errors are taken in stride. Thus, the mathematics of the guidance law are designed and optimized to achieve their best results if the input data is accurate respecting the missile and target states. The guidance law will produce erroneous control accelerations if the input data is not accurate.
As a general automatic control technique, it is known to smooth variations in input data in an effort to counteract noisy or poor input data. This reduces the responsiveness of the control. Another technique could be to reject input values that are out of an expected range, e.g., differing by too large a threshold from the values of the next previous values. If a large threshold is chosen, input errors can be introduced. If the threshold is small, valid input data might be discarded. There is little practical alternative other than to rely on the input data that is available.
Absolute input accuracy cannot be expected in the practical world. Some error in defining the missile and target states will be encountered and will affect the accuracy and success of the guidance law. The effect of an input error is that the guidance law seeks to adjust the missile flight path to intersect an erroneously predicted intercept point along the trajectory of the target. It would be advantageous if techniques could be employed to reduce adverse effects of input error.
If one assumes a guidance situation involving a constant target speed and target heading, as well as accurate input information and precise output control, a guided missile beginning on an arbitrary heading might be expected to require smaller and smaller lateral accelerations to make course corrections as the missile homes in more and more accurately on the correct point at which the missile trajectory will intersect the target trajectory. On the other hand, the target may change speed or heading, or the trajectory of the missile may be affected by external influences. If so, corrections are necessary. Moreover, as the missile nears the target and nears the point of intersection, a given lateral displacement distance perpendicular to a line of sight from the missile to the intersection point subtends an angular displacement that becomes greater than the same lateral displacement did from farther away, due to parallax.
Assuming that input errors may arise, for example due to random noise in assessing the current target heading, the guidance law responds nominally by applying acceleration in a direction perpendicular to a line of sight to the point of intersection, to alter the trajectory. Noise may cause the guidance control to alter the trajectory in a given direction away from an accurate heading leading toward target interception. Assuming that the error was momentary, subsequent control iterations mitigate the error as the guidance control alters the trajectory back in the other direction to more nearly accurate. Assuming that errors continue to arise randomly, the result can be unnecessary steering movements. In a proportionate line-of-sight control based on angular heading error, the amplitude of corrective steering accelerations and subsequent corrections may increase nearing the target. Unnecessary steering movements reduce the speed of the missile and detract from the mission.
It would be advantageous to provide a technique whereby a guidance system might distinguish between input data that is affected by noise and input data that is accurate, without discarding potentially valid input data. However, a variance in an input value due to noise may be difficult to distinguish from an effect to which the guidance system should respond, such as evasive movements of an independently guided target.
Missile guidance engineers have sought to mitigate adverse effects of noise leading to erroneous input values in various ways. One technique is to limit the frequency of missile guidance updates, which is a way to smooth the guidance data input. As discussed, this reduces the responsiveness of the guidance system to changes in input conditions such as evasive movements of the target.
Fuzzy logic techniques have been applied to the problem as disclosed in “Fuzzy Guidance in the Aerodynamic Homing Missiles,” Becan, Proceedings of the International Conference on Computational Intelligence, 2004, PP. 266-269. Fuzzy logic controls generally reduce the incidence of wide control swings by imposing rules, but in so doing also reduce the potential responsiveness of a control.
Another known technique is to designate a series of way points as intermediate destinations to intersect while approaching the target, rather than to re-compute the next destination at the same frequency as new and possibly noise-affected input data points become available to define the current target location, which would lead to frequent course corrections. This technique also renders the guidance system less accurate and responsive to the target, because the next previously computed way point may not be an optimal way point in view of changed conditions. The target may have deviated from its speed or heading since the way points were computed.
What is needed is a way to distinguish valid course correction needs so as to ignore, or at least damp, the response to erroneous input data that would lead to unnecessary or counter-productive course corrections. At the same time, the control scheme needs to maintain a sensitive and highly responsive reaction to valid course corrections to achieve intersection with the target trajectory as quickly and directly as possible.